The operation of cranes can produce conditions dangerous to both operators and persons or property nearby. The condition guarded against by the instant invention would result from the crane tipping over. Such event might occur should the crane boom approach vertical past a critical angle or should a load strike the boom. There have, therefore, been prior developments designed to warn the operator and other parties on the existance of a dangerous or near-dangerous condition. The closest art to the present invention, known to applicant, is that taught by U.S. Pat. No. 3,924,752 assigned to the same assignee as is this application. Such reference emits an audible signal if a load too closely approaches the boom, or if the boom reaches a critical angle. While this invention accomplishes the same goal, it does so in a greatly simplified manner. It further eliminates the need for exterior springs which might fail or become contaminated.